Efforts to understand the cause of 12C versus 13C isotope fractionation in plants during photosynthesis and post photosynthetic metabolism are frustrated by the lack of data on the intramolecular 13C-distribution in metabolites and its variation with environmental conditions. We have exploited isotopic carbon-13 nuclear magnetic resonance (13C NMR) spectrometry to measure the positional isotope
composition (d13Ci, ‰) in ethanol samples from different origins: European wines, liquors and sugars from C3, C4 and crassulacean acid metabolism (CAM) plants. In C3-ethanol samples, the methylene group was always 13C-enriched (~2‰) relative to the methyl group. In wines, this pattern was correlated with both air temperature and delta 18O of wine water, indicating that water vapour deficit may be a critical defining factor. Furthermore, in C4-ethanol, the reverse relationship was observed (methylene-C relatively 13C depleted), supporting the concept that photorespiration is the key metabolic process leading to the 13C distribution in C3-ethanol. By contrast, in CAM-ethanol, the isotopic pattern was similar to but stronger than C3-ethanol, with a relative 13C-enrichment in the methylene-C of up to 13‰. Plausible causes of this 13C-pattern are briefly discussed. As the intramolecular delta 13Ci -values in ethanol reflect that in source glucose, our data point out the crucial impact on the ratio of metabolic pathways sustaining glucose synthesis.